Imide-containing compounds are well-known to exhibit excellent heat resistance. It is also well-known in the art that polyimides provide excellent heat-resistant characteristics. Because of their excellent mechanical strength and heat resistance, polyimides have been widely used as the base material in making soft printed circuit board (PCB), packaging material for electronic components, wire varnish, etc. One of the major problems associated with polyimides, however, is that they are insoluble in most organic solvents, thus causing a great deal of difficulty in attempting to use solution technique for the fabrication of polyimide-based products. Typically, a precursor of polyimide, which is polyamic acid, is first synthesized and used in the fabrication process in making the polyimide-based products. After fabrication, the polyamic acid is subsequently converted to polyimide via an imidization reaction.
The need to involve polyamic acid in the polyimide fabrication processes presents a number of other problems such as the problem of storage of polyamic acid and the problems related to the strong corrosiveness of the acid, both problems have caused serious environmental concerns. Furthermore, a de-watering procedure is required to convert polyamic acid to polyimide in the imidization step, thus further complicating the fabrication process and introducing a waste disposal problem.
Another problem experienced with the polyimide resins is their lack of adhesion with metals, particularly with copper foils. Adhesion is an extremely important property in the use of polymers in electrical circuit applications because it determines, to a great extent, the overall reliability of the circuit. Failure in adhesion could, for example, lead to microvoids which could trap moisture and thereby lead to corrosion failure.
U.S. Pat. No. 3,541,038 ('038 patent) discloses a polyimidamide synthesized from the polycondensation of diisocyanate and trimellitic acid monomers. In the '038 patent, polyimidamide was prepared in the presence of a suitable organic solvent such as dimethylacetamide (DMAC), dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Best result was observed when NMP was used as solvent. Since NMP is a relatively expensive chemical, it is, therefore, desirable to develop a process that can make imide-containing polymer in a less expensive solvent system to thus reduce the cost of manufacturing therefor.
A number of U.S. patents also disclosed various types of polyamideimide compositions, for example: U.S. Pat. Nos. 3,716,519, 3,829,399, 4,061,623, 4,094,864, 4,332,925, 4,431,758, 4,599,383, 4,831,104, 4,927,906, 4,978,736, 4,987,197, 5,019,642, 5,028,688, 5,068,307, and 5,155,206. None of these patents, however, provides satisfactory solutions to the solubility and adhesion problems.